Visualizing Charge Carrier Dynamics in Transition Metal Dichalcogenide Nanoflakes Using Femtosecond Pump-Probe Microscopy

非技术性描述：半导体纳米结构在从太阳能转换到发光二极管的技术中发挥着越来越重要的作用。这些纳米结构是小晶体，可能是人类头发直径的1/1000，并且具有各种形状，包括小颗粒，长柔性线或薄片。然而，它们并不是完美的晶体。位于边缘和表面的原子与位于内部的原子的行为不同。此外，它们的小尺寸增加了它们的柔韧性，使电线能够弯曲，片材能够弯曲和折叠，这反过来又改变了它们的特性。 纳米结构的功能性通常由这些缺陷所支配，然而很难理清特定缺陷结构对材料行为的影响。该项目通过开发显微镜方法来应对这一挑战，该方法可以在短时间尺度上对单个纳米结构中的电子运动进行成像，并具有高空间分辨率。然后，这些方法被用来探索电子动力学在半导体纳米结构，只有几个原子层厚的空间变化。该项目的发现可能对从太阳能转换和催化到量子信息系统等新兴技术产生广泛影响。该研究还为研究生和本科生提供了培训机会，项目参与者正在与来自代表性不足背景的大学科学专业的学生合作，以发展科学交流技能。 技术说明：该项目正在推进泵浦-探测显微镜方法，并利用它们来探索电子弛豫动力学和电荷载流子传输如何在半导体纳米片上空间变化。纳米片的直径只有几微米，由3-30个过渡金属二硫属化物层（像纸一样）组成，这些层通过弱的货车德瓦尔斯力结合在一起。 单个纳米片用飞秒激光脉冲激发，该飞秒激光脉冲通过显微镜物镜聚焦到衍射极限点，从而在结构的局部区域中促进电子从价带到导带。然后，通过在时间上延迟的第二聚焦激光脉冲探测激发的电子数。探测脉冲通过多光子吸收过程从结构中激发电子，并使用速度图成像光电子能谱测量它们的动量。射出的电子的动量分布提供了一个直接的窗口，通过该窗口可以监测能量弛豫。在纳米片中的不同位置（例如边缘与中心）处进行的实验显示了不同的结构特征如何影响电荷载流子弛豫和复合。显微镜还可以在一个位置激发纳米片，并在另一个位置探测它，从而在电子移动通过结构时直接观察电子。 研究活动开发的显微镜工具广泛适用于各种纳米材料，学生们正在获得建造复杂仪器的经验。研究生和本科生还与北卡罗来纳州大学校长科学学者计划的新生合作，通过开发突出项目研究成果的短视频，探索向广大观众解释复杂科学发现的重要性和挑战。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准。

Nontechnical description: Semiconductor nanostructures are playing an ever-increasing role in technologies that range from solar energy conversion to light emitting diodes. These nanostructures are small crystals, perhaps 1/1000 the diameter of a human hair, and come in a variety of shapes, including small particles, long flexible wires, or thin sheets. They are not perfect crystals, however. Atoms at the edges and surfaces act differently from those located in the interior. In addition, their small dimensions increase their flexibility, enabling wires to bend, and sheets to buckle and fold, which in turn changes their properties. Nanostructure functionality is oftentimes dominated by these imperfections, yet it is difficult to disentangle the influence of specific defect structures on material behavior. This project is addressing this challenge by developing microscopy methods that can image the movement of electrons in individual nanostructures on short time scales, and with high spatial resolution. These methods are then used to explore the spatial variation in electronic dynamics across semiconductor nanostructures that are only a few atomic layers thick. The project's discoveries could have broad implications for emerging technologies ranging from solar energy conversion and catalysis to quantum information systems. The research also provides training opportunities for graduate and undergraduate students, and the project participants are working with college science majors from underrepresented backgrounds to develop skills in scientific communication. Technical description: The project is advancing pump-probe microscopy methods and using them to explore how the electronic relaxation dynamics and charge carrier transport vary spatially across semiconductor nanoflakes. The nanoflakes are a few microns across and consist of 3-30 transition metal dichalcogenide layers (like sheets of paper) held together by weak van der Waals forces. Individual nanoflakes are excited with a femtosecond laser pulse that is focused to a diffraction limited spot by a microscope objective, promoting electrons from the valence band to the conduction band in a localized region of the structure. The excited electron population is then probed by a second focused laser pulse that is delayed in time. The probe pulse ejects electrons from the structure through a multiphoton absorption process, and their momentum is measured using velocity-map imaging photoelectron spectroscopy. The momentum distribution of the ejected electrons provides a direct window through which the energetic relaxation can be monitored. Experiments performed at different locations in the nanoflake (e.g. edge vs. center) show how different structural features affect the charge carrier relaxation and recombination. The microscope can also excite the nanoflake in one location, and probe it in another, enabling direct visualization of the electrons as they move through the structure. The microscopy tools developed by the research activities are broadly applicable to a wide range of the nanomaterials, and the students are gaining experience in the construction of sophisticated instrumentation. The graduate and undergraduate students are also working with entering freshmen in the University of North Carolina Chancellor's Science Scholars Program to explore the importance and challenges of explaining complex scientific discoveries to broad audiences through the development of short videos that highlight the project's research results.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.